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| United States Patent |
5,614,315
|
|
Kondo
, et al.
|
March 25, 1997
|
Heat-shrinkable multi-layer polyolefin films
Abstract
A heat shrinkable multilayer film which includes two outer layers
consisting essentially of a polyethylene resin, and one or more inner
layers interposed between the outer layers, wherein at least one of the
inner layers comprises a blend of a polypropylene resin and a
substantially linear ethylene/alpha-olefin copolymer, wherein said
copolymer has a molecular weight distribution Mw/Mn of not greater than
about 2 and a melt flow ratio I.sub.10 /I.sub.2 of not smaller than 7.0.
| Inventors:
|
Kondo; Kazuo (Marugame, JP);
Tada; Teruo (Marugame, JP);
Wano; Toyoki (Zentsuji, JP);
Uehara; Hideki (Takamatsu, JP);
Tsuchida; Tomohisa (Kagawa-ken, JP)
|
| Assignee:
|
Okura Industrial Co., Ltd. (Amagasaki, JP)
|
| Appl. No.:
|
375829 |
| Filed:
|
January 20, 1995 |
| Current U.S. Class: |
428/332; 428/34.9; 428/36.91; 428/515; 428/516; 428/520 |
| Intern'l Class: |
B32B 027/08; B32B 027/32; B65B 053/02 |
| Field of Search: |
428/516,34.9,519,523,515,332,35.7,36.91,34.7,520
|
References Cited
U.S. Patent Documents
| 4820557 | Apr., 1989 | Warren et al. | 428/34.
|
| 5001016 | Mar., 1991 | Kondo et al. | 428/516.
|
| 5279872 | Jan., 1994 | Ralph | 428/34.
|
| Foreign Patent Documents |
| 0600425A1 | Jun., 1994 | EP.
| |
| 0634443A3 | Jan., 1995 | EP.
| |
| WO94/07954 | Apr., 1994 | WO.
| |
Primary Examiner: Thibodeau; Paul J.
Assistant Examiner: Tarazano; Donald Lawrence
Attorney, Agent or Firm: Armstrong, Westerman, Hattori, McLeland & Naughton
Claims
What is claimed is:
1. A heat-shrinkable multilayer film comprising two outer layers each
consisting essentially of a polyethylene resin, and one or more inner
layers interposed between said outer layers, wherein at least one of said
inner layers is formed of a blend of a polypropylene resin with a linear
ethylene/alpha-olefin copolymer, wherein said copolymer has a molecular
weight distribution Mw/Mn of not greater than about 2 and a melt flow
ratio of I.sub.10 /I.sub.2 of not smaller than 7.0.
2. A multilayer film as claimed in claim 1, wherein said
ethylene/alpha-olefin copolymer is an ethylene/octene-1 copolymer.
3. A multilayer film as claimed in claim 1, wherein said
ethylene/alpha-olefin copolymer has a density of 0.890-0.910 g/cm.sup.3.
4. A multilayer film as claimed in claim 1, wherein said
ethylene/alpha-olefin copolymer has a melt index of 0.8-2.0 g/10 min.
5. A multilayer film as claimed in claim 1, wherein the weight ratio of
said ethylene/alpha-olefin copolymer to said polypropylene resin is 20:80
to 60:40.
6. A multilayer film as claimed in claim 1, wherein the weight ratio of
said ethylene/alpha-olefin copolymer to said polypropylene resin is 30:70
to 50:50.
7. A multilayer film as claimed in claim 1, wherein said polyethylene resin
is at least one member selected from the group consisting of linear low
density polyethylenes and very low density linear polyethylenes.
8. A multilayer film as claimed in claim 1, wherein the number of said
inner layers is three, wherein said three inner layers consist of two
intermediate layers and a core layer interposed between said two
intermediate layers and wherein each of said intermediate layers is formed
of said blend.
9. A multilayer film as claimed in claim 8, wherein said core layer is
formed of a mixture of said polyethylene resin with said blend.
10. A multilayer film as claimed in claim 1, wherein the film has a total
thickness of a thickness of 10-35 .mu.m.
11. A multilayer film as claimed in claim 1, and having a shrinkage value
of 15-30% at 90.degree. C. in both machine and transverse directions.
12. A multilayer film as claimed in claim 1, cross-linked by irradiation
with actinic radiation.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a multilayer film and, more
specifically, to an easily producible, heat-shrinkable polyolefin
multilayer film having excellent heat-shrinkability and tearing strength.
2. Background of Invention
In order for a heat-shrinkable film to easily provide a tight fit package,
the film is required to have both high heat-shrinkability and high
mechanical strengths, such as a tearing strength and resistance to
breakage during heat sealing and during use. In addition, the film with
such desired properties must be produced without difficulty.
A heat-shrinkable film in a tubular form is generally produced by a process
in which a thermoplastic resin melt is extruded through a die to form a
tubular preform, the preform being rapidly cooled with water, then
reheated to a temperature lower than the melting point thereof but higher
than the softening point or glass transition point thereof and stretched
by a blown bubble method. The resulting biaxially oriented film when
heated shows a property to return to the original dimensions.
The characteristics of the heat-shrinkable film, therefore, depend not only
upon the physical properties of the raw material thermoplastic resins but
also upon stretching conditions such as the stretching temperature and the
drawing ratio. For example, a heat-shrinkable film formed mainly of a
polypropylene having a high melting point, a high degree of crystallinity
and good stretching processability is not good in low temperature
shrinkability and has a low maximum shrinkage value, although the tensile
strength and shrinkage stress of the film are high. In contrast, a
heat-shrinkable film formed mainly of a resin having a low melting point
and low degree of crystallinity, such as a low density polyethylene
(hereinafter referred to as LDPE), a linear low density polyethylene
(hereinafter referred to as LLDPE), a very low density linear polyethylene
(hereinafter referred to as VLDPE) or an ethylene/vinyl acetate copolymer
(hereinafter referred to as EVA), exhibits low temperature shrinkability,
a low shrinkage stress and excellent heat sealability. However, since such
a low melting point resin is stretchable only in a narrow temperature
range, it is necessary to strictly control the stretching temperature or
to cross-link the resin by irradiation with an electron beam.
To cope with the foregoing problems, there have been proposed various kinds
of heat-shrinkable films in which two or more different resins are used in
combination as a blend and/or as a laminate.
U.S. Pat. Nos. 4,194,039 and 4,229,241 disclose that a heat-shrinkable,
three layer film composed of two skin layers of an ethylene/propylene
copolymer and a core layer of a blend of an ethylene/vinyl acetate
copolymer with an ethylene/butylene copolymer shows such a shrinkage
stress, clarity, sealing temperature, a wide temperature range for
shrinking and a tearing strength that would not be obtainable with a
polyolefin single layer film.
U.S. Pat. No. 4,532,189 discloses a heat-shrinkable multilayer film having
a core layer of LLDPE or a linear medium density polyethylene (hereinafter
referred to as LMDPE) and showing improved mechanical strengths.
U.S. Pat. No. 4,551,380 discloses a heat-shrinkable three layer film having
a core layer of a blend of LLDPE, LMDPE and EVA and exhibiting good heat
sealability and shrinkability.
U.S. Pat. No. 4,820,557 discloses a high abuse-resistant, heat-shrinkable
film including at least one layer of an ethylene/alpha-olefin copolymer
having a density of 0.935 g/cm.sup.3 or less and a lower I.sub.10 /I.sub.2
ratio in comparison with the corresponding ethylene/alpha-olefin which has
the same comonomer, the same density and the same melt index at
190.degree. C. and 2.16 kg loading.
U.S. Pat. No. 5,059,481 discloses a puncture-resistant, heat-shrinkable
film which includes VLDPE having a density of 0.86-0.91 g/cm.sup.3 and a
1% modulus of below 140,000 kPa and which shows a shrinkage value of
30-50%.
U.S. Pat. No. 4,837,084 discloses that a heat-shrinkable multilayer film
containing a layer of a copolymer of ethylene and an alpha-olefin with six
or more carbon atoms having a density of 0.910 g/cm.sup.3 or less and a
melt index of 2 or less exhibits excellent heat-shrinkability, orientation
speed properties and abuse-resistant properties.
U.S. Pat. No. 4,302,557 discloses a cold drawn film having excellent low
temperature shrinkability and formed of a blend of (A)+(B), (B)+(C) or
(A)+(B)+(C), wherein (A) is a non-rigid polyolefin resin such as EVA, (B)
is an ethylene/alpha-olefin elastomer and (C) is a rigid polyolefin resin
such as polypropylene.
U.S. Pat. No. 5,272,016 discloses that a heat-shrinkable multilayer film
having opposite outer layers each formed of a blend of VLDPE (density of
0.914 g/cm.sup.3 or less) with an ethylene/alpha-olefin plastomer (density
of 0.90 g/cm.sup.3 or less) is suited for overwrapping trays.
U.S. Pat. Nos. 4,923,722, No. 5,001,016 and 4,948,657 disclose that a
heat-shrinkable multi-layer film having a core or inner layer of a blend
of polypropylene with VLDPE and outer layers each formed of polypropylene
shows good low temperature shrinkability. The tearing strength of such a
layer is 4.0-11.6 kg/cm (tearing load is 8.0-23.2 g at 20 .mu.m) which is
lower than that of a heat-shrinkable film formed mainly of LLDPE, VLDPE or
EVA.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
polypropylene-containing, heat-shrinkable film which has good orientation
processing characteristics and an improved tearing strength and which is
useful for the heat shrinking packaging of various articles.
Another object of the present invention is to provide a heat-shrinkable
film of the above-mentioned type which shows good shrinkage
characteristics at a low temperature and which shows a high maximum
shrinkage value.
It is a further object of the present invention to provide a
heat-shrinkable film of the above-mentioned type which is excellent in
slippage and anti-blocking properties and which gives easily openable
packages.
It is yet a further object of the present invention to provide a
heat-shrinkable film of the above-mentioned type which exhibits excellent
heat-sealability and optical characteristics.
In accomplishing the foregoing objects, the present invention provides a
heat-shrinkable multilayer film including two outer layers each formed of
a polyethylene resin, and one or more inner layers interposed between the
outer layers, wherein at least one of the inner layers is formed of a
blend of a polypropylene resin with a substantially linear
ethylene/alpha-olefin copolymer having a molecular weight distribution
Mw/Mn of not greater than about 2 and a melt flow ratio I.sub.10 /I.sub.2
of not smaller than 7.0.
Other objects, features and advantages of the present invention will become
apparent from the detailed description of the preferred embodiments of the
invention to follow.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The heat-shrinkable film according to the present invention is a laminate
having two outer layers each formed of a polyethylene resin and one or
more inner layers interposed between the outer layers. Thus, the
heat-shrinkable film is a multilayer film having three or more layers.
It is important that at least one of the inner layers should be formed of a
blend of a polypropylene resin with a substantially linear
ethylene/alpha-olefin copolymer having a molecular weight distribution
Mw/Mn of not greater than about 2 and a melt flow ratio I.sub.10 /I.sub.2
of not smaller than 7.0.
The molecular weight distribution Mw/Mn is a ratio of the weight average
molecular weight to the number average molecular weight of the copolymer,
while the melt flow ratio I.sub.10 /I.sub.2 is a ratio of the melt index
at the 10 kg loading to the melt index at the 2.16 kg loading at
190.degree. C. as per ASTM D1238. The substantially linear
ethylene/alpha-olefin copolymer having the above specific Mw/Mn and
I.sub.10 /I.sub.2 will be referred to in the present specification as
"polyolefin plastomer".
The polyolefin plastomer preferably has a density of about 0.890-0.935
g/cm.sup.3, more preferably 0.890-0.910 g/cm.sup.3, and a melt index of
0.8-2.0 g/10 min at 2.16 kg loading 190.degree. C. as per ASTM D1238. The
alpha-olefin constituting part of the polyolefin plastomer preferably has
4-8 carbon atoms, more preferably 8 (octene-1). Plastomers commercially
available as AFFINITY (trademark of The Dow Chemical Company) may be
suitably used as the polyolefin plastomer. The polyolefin plastomer is
distinguished from LLDPE, VLDPE and an ethylene/alpha-olefin elastomer in
Mw/Mn and/or I.sub.10 /I.sub.2.
LLDPE is defined herein as an ethylene/alpha-olefin (with 4-8 carbon atoms)
copolymer having a density of greater than 0.910 g/cm.sup.3 but not
greater than 0.925 g/cm.sup.3. "DOWLEX 2045 A" (product of The Dow
Chemical Company) is an example of LLDPE. VLDPE is defined herein as an
ethylene/alpha-olefin (with 4-8 carbon atoms) copolymer having a density
of 0.890-0.910 g/cm.sup.3. Examples of VLDPE include "VL103" (product of
Sumitomo Chemical Co., Ltd.) and "EXACT 3028" (product of Exxon Chemical
Co.). The ethylene/alpha-olefin elastomer is defined herein as an
ethylene/alpha-olefin (with 3-8 carbon atoms) copolymer having a structure
similar to LLDPE or VLDPE but having a lower degree of crystallinity (less
than 30%) than LLDPE or VLDPE. The elastomer, therefore, does not have a
melting point or has a very low melting point. "TAFMER A1085" (product of
Mitsui Petrochemical Co., Ltd.) is an example of the elastomer.
Physical properties of typical polyethylenes are given below.
TABLE 1
__________________________________________________________________________
AFNTY*1
LLDPE*2
VL103*3
EXACT*4
ELSTM*5
Resin olefin octene-1
octene-1
butene-1
butene-1
butene-1
__________________________________________________________________________
Density (g/cm.sup.3)
0.902 0.920 0.905
0.900 0.885
Melting Point (.degree.C.)
100 121 117 92 71
Melt Index (g/10 min)
1.0 1.0 015 1.2 1.4
I.sub.10 /I.sub.2
8.9 7.8-8.0
10.2 5.5 4.8
Mw/Mn 2.0 3.5-3.8
4.7 2.0 2.0
Tensile Strength (kg/cm.sup.2)
265 250 340
MD 500 605
TD 265 510
Elongation (%) 1,000 900 710
MD 570 590
TD 560 680
__________________________________________________________________________
*1: Polyolefin plastomer; AFFINITY PL1880 (Dow Chemical)
*2: LLDPE; DOWLEX 2045 (Dow Chemical)
*3: VLDPE; VL103 (Sumitomo Chemical)
*4: VLDPE; EXACT 3028 (Exxon Chemical), single site catalyst
*5: Ethylene/alphaolefin elastomer; TAFMER A1085 (Mitsui Petrochemical)
The polypropylene resin used in conjuction with the polyolefin plastomer
may be a propylene homopolymer or a copolymer of propylene with an olefin.
A polypropylene resin having a melt index of 1-4 g/10 min. is preferably
used. Illustrative of suitable polypropylene resins are ethylene/propylene
copolymers, propylene/butene-1 copolymers, ethylene/propylene/butadiene
terpolymers.
The weight ratio of the polyolefin plastomer to the polypropylene resin in
the blend is preferably 20:80 to 60:40, more preferably 30:70 to 50:50.
When the amount of the polypropylene resin is smaller than the above
preferred range, the stretching processing properties of the film is
lowered so that it is necessary to perform a cross-linking treatment by
irradiation with an electron beam or the like radiation before stretching.
Too large an amount of the polypropylene resin will adversely affect the
tearing strength of the heat-shrinkable film.
Any polyethylene resin may be used for each of the two outer layers.
Illustrative of suitable polyethylene resins are LDPE, LLDPE, VLDPE, EVA
and polyolefin plastomers. Especially preferred is the use of LLDPE,
VLDPE, polyolefin plastomers or a mixture thereof, for reasons of improved
stretch processing properties, tearing strength, heat-shrinkability,
surface slippage and Young's modulus.
The heat-shrinkable multilayer film according to the present invention may
include one or more inner layers in addition to the blend resin layer or
layers of the above-described blend of the polypropylene resin with the
polyolefin plastomer. Such an additional layer or layers may be formed of
a polyethylene resin or a mixture of a polyethylene resin with an adhesive
resin. For reasons of cost reduction and improved interlayer adhesion, it
is preferred that the additional layer or layers be formed of a mixture of
the outer layer resin with the blend layer resin, which resins are
recovered as trimmings or chips during the course of the preparation of
the heat-shrinkable film.
For example, in one preferred embodiment, the heat-shrinkable multilayer
fill is a five layer laminate, wherein the inner layers consist of two
intermediate layers and a core layer interposed between the two
intermediate layers, wherein each of the intermediate layers is formed of
the blend of the polypropylene rash with the polyolefin plastomer and
wherein the core layer is formed of a mixture of the polyethylene resin
obtained from the outer layers with the blend obtained from the
intermediate layers.
The thickness of the heat-shrinkable multilayer fill is preferably about
10-35 .mu.m. The proportion in thickness of respective layers constituting
the film varies with the intended use thereof and the kinds of the raw
material resins. In the case of a five layer film consisting of (outer
layer)/(intermediate layer)/(core layer)/(intermediate layer)/(outer
layer), the proportion in thickness of respective layers is preferably
(10-20%)/(15-30%)/(20-40%)/(15-30%)/(10-20%), wherein the percentages are
based on the total thickness of the film, for reasons of optimum tearing
strength and stability in production process. The heat-shrinkable
multilayer film preferably has a shrinkage value of 15-30% at 90.degree.
C. in both machine and transverse directions.
The multilayer film according to the present invention may be produced as
follows. The raw material resins are charged in respective extruders,
melted and coextruded through a multilayer die to form a tubular film. The
extruded film is rapidly cooled with water. The resulting tube is
preheated in a preheating zone and is fed to a heating zone where it is
heated to a stretching temperature. Air is fed into the heated tube so
that the tube is inflated and biaxially oriented. The stretched tube is
then fed to an annealing zone where it is heated to prevent spontaneous
shrinkage thereof during transportation and storage and to uniformize
internal strain thereof.
The heat-shrinkability of the thus obtained heat-shrinkable film depends on
the stretching temperature and drawing ratio. Generally, the
heat-shrinkable film obtained at a stretching temperature (film
temperature) of 90.degree.-120.degree. C. with a drawing ratio of 3-4 in
both the machine and transverse directions shows a shrinkage value at
90.degree. C. of at least 15% and a maximum shrinkage value of 50% or more
in both the machine and transverse directions.
If desired, the film is subjected to a cross-linking treatment by
irradiation with an electron beam or the like actinic radiation before or
after the biaxial stretching to impart a heat resistance to the film.
The following examples will further illustrate the present invention.
Examples 1-5 and Comparative Examples 1-5
Five-layered films each including a core layer, two intermediate layer
sandwiching the acre layer, and two outer layers were produced using the
resins selected from Resins A-J and shown in Table 2. Resin A-J are as
follows:
Resin A: Polyolefin plastomer, Affinity PL1880 (Dow Chemical; Details are
shown in Table 1);
Resin B: Polyolefin plastomer, Affinity PF1140 (Dow Chemical; olefin:
octene-1; density: 0.895 g/cm.sup.3 ; MI: 1.6 g/10 min; I.sub.10 /I.sub.2
: 10.1, Mw/Mn: 2.0);
Resin C: Polyolefin plastomer, Affinity PL1840 (Dow Chemical; olefin:
octene-1; density: 0.908 g/cm.sup.3 ; MI: 1.0 g/10 min; I.sub.10 /I.sub.2
: 9.0, Mw/Mn: 2.0);
Resin D: LLDPE; Dowlex 2045 (Dow Chemical; Details are shown in Table 1);
Resin E: LLDPE; FZ102 (Sumitomo Chemical; olefin: hexene-1; density: 0.912
g/cm.sup.3 ; MI: 0.8 g/10 min; I.sub.10 /I.sub.2 : 8.9; Mw/Mn: 4.1);
Resin F: LLDPE; Attane 4201 (Dow Chemical; olefin: octene-1; density: 0.912
g/cm.sup.3 ; MI: 1.0 g/10 min; I.sub.10 /I.sub.2 : 8.2; Mw/Mn: 3.8);
Resin G: VLDPE; VL103 (Sumitomo Chemical; Details are shown in Table 1);
Resin H: VLDPE; EXACT 3028 (Exxon Chemical; Details are shown in Table 1);
Resin I: polypropylene; WF905E (Sumitomo Chemical; ethylene/propylene
copolymer; ethylene content: 4-7%; density: 0.890 g/cm.sup.3 ; MI: 3.9
g/10 min);
Resin J: polypropylene; S131 (Sumitomo Chemical; ethylene/propylene
copolymer; ethylene content: 4.7%; density: 0.890 g/cm.sup.3 ; MI: 1.2
g/10 min)
The outer layer resin, intermediate layer resin and core layer resin were
charged in respective extruders and coextruded through a circular die
(diameter: 180 mm) at an extrusion rate of 50 kg/hour and at a die
temperature of 210.degree. C. to form a tubular extrudate. The extrudate
was rapidly cooled to obtain a tubular preform having a diameter of 180 mm
and a thickness of 220 .mu.m. The preform was preheated, then heated to
about 100.degree. C. and, thereafter, biaxially stretched simultaneously
in both the machine and transverse directions. The drawing ratio
(MD.times.TD) was 3.9.times.3.4. The stretched tube was reheated to effect
slight shrinkage (to a degree of 5-10%), thereby obtaining a
heat-shrinkable film having a total thickness of 19 .mu.m. The thickness
proportion of (outer layer)/(intermediate layer)/(core
layer)/(intermediate layer)/(outer layer) was
(15%)/(17.5%)/(35%)/(17.5%)/(15%) in the case of Examples 1-4 and
Comparative Examples 2-5, (10%)/(25%)/(30%)/(25%)/(10%) in the case of
Example 5 and (20%)/(12.5%)/(35%)/(12.5%)/(20%) in the case of Comparative
Example 1. The stretching processability (evaluated according to the
following ratings: A . . . good, B . . . no good) and physical properties
of the thus obtained films are shown in Table 3.
TABLE 2
__________________________________________________________________________
Example (Comp. Ex.)
1 2 3 4 5 (1)
(2)
(3)
(4)
(5)
__________________________________________________________________________
Outer Layer
Resin C 100 100
100
Resin D 100
100 100 100
100
100
100
Intermediate Layer
Resin A 30 30 30 40
Resin B 30
Resin E 30
Resin F 30
Resin G 50
Resin H 30
Resin I 70 70 70 60 100
70 50 70
Resin J 70 70
Core Layer
Resin A 16 16 16 28
Resin B 16
Resin C 46 30 60
Resin D 46 46 46 46 46 46 46
Resin E 16
Resin F 16
Resin G 16
Resin H 16
Resin I 38 38 38 42 40
38 38 38
Resin J 38 38
__________________________________________________________________________
TABLE 3
__________________________________________________________________________
Example (Comparative Example)
1 2 3 4 5 (1) (2) (3) (4) (5)
__________________________________________________________________________
Stretching Processability
A A A A A A A A A B
Haze (%) 3.6 3.4 2.6 3.5 2.8 2.6 3.8 3.3 3.9 3.3
Young's Modulus
3360/
3450/
2730/
3000/
2840/
4150/
3200/
3290/
3120/
2810/
MD/TD (kg/cm.sup.2)
3530
4000
2540
3120
2560
4530
3300
3440
3250
2790
Tearing Load MD/TD (g)
26/40
18/23
25/38
21/20
21/32
9/11
10/12
10/20
13/20
8/16
Slippage MD/TD (.times.10.sup.-2)
20/14 17/13 76/22 18/14 21/14
Shrinkage Value MD/TD
at 90.degree. C.
20/24
19/24
21/25
18/20
24/25
22/24
18/20
19/20
22/24
20/22
at 100.degree. C.
36/38
35/37
37/40
34/36
38/41
38/40
34/36
35/37
36/38
35/37
at 110.degree. C.
50/52
50/53
52/54
49/51
54/55
46/48
49/50
50/51
50/52
49/51
at 120.degree. C.
61/60
61/60
61/61
60/60
64/63
57/57
61/60
59/60
61/60
59/59
__________________________________________________________________________
In Table 3, the tearing load is measured prior to the heat shrinkage of the
film. The haze is measured in accordance with ASTM D1003, the Young's
modulus is measured in accordance with ASTM D638, the tearing load is
measured in terms of Elmendorf tearing strength in accordance with ASTM
D1922, the slippage is measured in accordance with ASTM D1894 and the
shrinkage is measured in accordance with a modified method of ASTM D2732.
As will be appreciated from the results summarized in Table 3, the
heat-shrinkable films according to the present invention (Examples 1-5)
have a high tearing strength, good stretching processability and excellent
temperature shrinkability. The heat-shrinkable film of Comparative Example
1 in which each of the intermediate layers is formed only of the
polypropylene resin has much lower tearing strength in comparison with
that of Example 3 even though the thickness of the intermediate layers is
smaller in Comparative Example 1. In the films of Comparative Examples 2
and 3 in which LLDPE is substituted for the polyolefin plastomer in
Examples 1 and 2, no improvement of the tearing strength is obtainable. In
the film of Comparative Example 4 in which VLDPE having a density of 0.905
g/cm.sup.3 (similar to the polyolefin plastomer) is used, no improvement
of the tearing strength is obtainable. In the film of Comparative Example
5 in which VLDPE having a density and a Mw/Mn ratio similar to those of
the polyolefin plastomer is used, not only the tearing strength is lowered
but also the stretching processability becomes poor.
The invention may be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. The present
embodiments are therefore to be considered in all respects as illustrative
and not restrictive, the scope of the invention being indicated by the
appended claims rather than by the foregoing description, and all the
changes which come within the meaning and range of equivalency of the
claims are therefore intended to be embraced therein.
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